Evaporative Cooling Nets Improved Production

While certainly an ancient technology, the utilization of geothermal resources has expanded in recent years, with engineers creating new technologies that allow facilities to tap miles below the Earth's surface to retrieve geothermal energy.Geothermal power plants, for example, use steam, heat or hot water from reservoirs in the earth to provide the force that spins the turbine generators...

Staff

02/01/2002

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While certainly an ancient technology, the utilization of geothermal resources has expanded in recent years, with engineers creating new technologies that allow facilities to tap miles below the Earth's surface to retrieve geothermal energy.

Geothermal power plants, for example, use steam, heat or hot water from reservoirs in the earth to provide the force that spins the turbine generators that produce electricity. The water is used and then returned down an injection well into the reservoir to be reheated, to maintain pressure and sustain the reservoir.

A mammoth project

One example of a geothermal power plant is the Mammoth Pacific geothermal facilities, located in California's Sierra Nevada mountains and fueled by the Casa Diablo Hot Springs. Built in 1984, the plant generates 32 net megawatts of renewable electricity.

"We pump water ranging in temperature from 300°F to 400°F to the surface through the production well where it is passed into a heat exchanger," says Bob Sullivan, general manager at Mammoth Pacific. "Our facilities consist of 12 production wells and nine injection wells. A total of eight single-stage, radial-flow gas expanders are used in the process."

The Mammoth Pacific facilities pump the hot water from more than 300 feet underground, and its heat is transferred into a second liquid—isobutane—that boils at a lower temperature than water.

When heated, the isobutane flashes to vapor. Like steam, it expands and spins the turbine blades, generating electricity. The isobutane vapor runs through large fin fan condensers, which are air-cooled heat exchangers similar to those found in condensing units for commercial and residential air conditioners. They draw large amounts of ambient air across their fins, thereby rejecting the heat from the isobutane directly into the air, instead of dissipating heat into water and then transferring that heat to the air—as with shell-and-tube heat exchangers and wet cooling tower systems.

Once the air-cooled fin fan rejects heat from the vapor and returns it to a liquid state, the process continues through another cycle. In this closed-loop system, there are no emissions and no fuel is required.

Improving efficiency

Last summer, with energy concerns in mind, Mammoth Pacific sought ways to improve efficiencies and generate more power.

"Our goal was to regain power at the plant by using materials to filter water to cool the fin fans," says Sullivan. "During the research phase of our project, we discovered that an evaporative cooling technology could meet our increased power needs."

Because the geothermal facilities are air-cooled, the efficiency changes depending on the ambient temperatures. As a result, when the air heats up during the summer, the air coolers become less efficient. Evaporative cooling systems are designed to substantially reduce the ambient air temperatures during these hot summer days.

The principle is quite simple. Water is applied to the top of the evaporative cooling media and allowed to trickle down. It spreads out over the extensive surface and mixes with the air, which is passed through the corrugations. When the water evaporates, it requires energy to pass from the liquid to the gaseous stage. The water vapor absorbs this heat from the air, thereby lowering the air temperature as the relative humidity is increased.

However, in order to utilize evaporative cooling systems, Sullivan realized that they would have to pump additional water to the plant to lower the ambient air temperature surrounding the condensers at the plant.

"To pump the tertiary treated water to the facilities, we built a 2-1/2 mile long pipeline to provide us with up to 1.15 million gallons of water per day for industrial cooling purposes," Sullivan explains. "That means that up to 600 gallons of water comes through the pipe every minute."

Sullivan tested two different evaporative cooling systems for use at the Mammoth plant—including a misting system—and decided on a conventional system where water is distributed over the cooling pad as air is drawn through the media, hence cooling the air toward the wet-bulb temperature. In addition, the media pads chosen were developed for applications requiring UL-900 Class II fire rating or compliance with NFPA codes—an important factor when dealing with flammable liquids such as isobutane.

Quantifying the benefits

Installed last summer, Sullivan has quickly been able to evaluate the benefits of the evaporative cooling system.

"We improved our power output by as much as 20% and averaged 10% to 15% more power with the addition of the evaporative cooling units during the test," he says. "Without these systems, we previously could not condense the isobutane to liquid as efficiently so power would drop. By using coolers in front of the fin fans, the ambient temperature dropped by as much as 25°F."

In fact, the company is now considering evaporative cooling for the other fin fan coolers.